Implantable cardiac stimulator

ABSTRACT

An implantable cardiac stimulator, comprises a ventricular detection unit which is adapted to be connected to an intracardiac electrode and to record and detect ventricular events, a ventricular stimulation unit which is adapted to be connected to a ventricular electrode and to produce ventricular stimulation pulses for delivery to the ventricle of a heart, and a control unit connected to the ventricular detection unit and to the ventricular stimulation unit and adapted to actuate the ventricular stimulation unit in a VVI mode in ventricle-inhibited fashion in such a way that a ventricular stimulation pulse is triggered at a moment predetermined by a stimulation rate if it is not inhibited by detection of a natural ventricular contraction by the ventricular detection unit within a predetermined time window. The control unit is adapted to predetermine a stimulation rate, which is higher than intrinsic rate appropriate to the physiological demand.

BACKGROUND OF THE ART

The invention concerns an implantable cardiac stimulator, in particulara cardiac pacemaker or cardioverter/defibrillator (ICD). The cardiacstimulator includes a ventricular detection unit which is to beconnected to an intracardiac electrode and is adapted to register anddetect ventricular events. The cardiac stimulator also includes aventricular stimulation unit which is to be connected to a ventricularelectrode and is adapted to produce ventricular stimulation pulses fordelivery to the ventricle of a heart. The cardiac stimulator furtherincludes a control unit which is connected to the ventricular detectionunit and to the ventricular stimulation unit and is adapted to actuatethe ventricular stimulation unit in the VVI mode (ventricle-inhibited)in such a way that a ventricular stimulation pulse is triggered at amoment in time predetermined by a stimulation rate, if it is notinhibited by detection of a natural ventricular contraction by means ofthe ventricular detection unit within a predetermined time window. Thepredetermined time window is frequently referred to as the escapeinterval.

Implantable cardiac pacemakers which are designed for the stimulation ofa human heart in a ventricle-inhibited mode are basically known. Thevarious stimulation and sensing modes are generally uniformly identifiedby a three-letter code of which the first letter denotes the stimulationlocation (V=ventricle, A=atrium, D=ventricle and atrium), the secondletter denotes the sensing location (V=ventricle, A=atrium, D=ventricleand atrium) and the third letter denotes the operating mode(I=inhibited, T=triggered, D=both inhibited and also triggered).Particularly for dual-chamber cardiac pacemakers in the DDD mode, it isalso known to effect ventricular stimulation in synchronous relationshipwith an atrial heart rate which is as natural as possible. If a healthynatural heart rate is not to be found in the atrium, for example in thecase of atrial tachycardia or atrial fibrillation, basicallyatrium-synchronous cardiac pacemakers frequently involve mode switchingfrom atrium-synchronous ventricular stimulation to atrium-asynchronousstimulation in the VVI mode if a detected atrial rate is outsideadmissible limits.

It is further known for ventricular tachycardias to be treated in thecontext of a cardioversion therapy by stimulation at a stimulation ratewhich is above the tachycardia rate. It is possible in that way to breaka series of supraventricular and ventricular tachycardias. The aim is inparticular to interrupt re-entry circles by an ectopic stimulusformation center being depolarized prematurely by stimulation before atrinsic stimulus can become effective. The stimulation frequency foroverdrive stimulation is generally so selected that it is between tenand fifteen pulses per minute higher than the tachycardia to beterminated. The state of the art provides a summary of approaches interms of how to react to existing tachycardias. One reaction to atrialtachycardia is for example mode switching into the VVI mode and as areaction to ventricular tachycardia it is possible to provide fortriggering overdrive stimulation as a cardioversion therapy.

It is an aspect of the invention to provide a cardiac stimulator withwhich the occurrence of ventricular tachycardias can be prevented as faras possible from the outset.

SUMMARY OF THE INVENTION

In accordance with that invention the aspect is attained by animplantable cardiac stimulator of the kind set forth in the opening partof this specification, in which the control unit is adapted topredetermine a stimulation rate which is higher than an intrinsic rateappropriate to the physiological demand. That predetermined stimulationrate is preferably variable and is also referred to hereinafter as theoverstimulation rate in order to make it clear that the overstimulationrate is higher than a rate adapted to the physiological demand.

Unlike all known cardiac pacemakers, the cardiac pacemaker according tothe invention is thus intended to operate in a kind of overdrive modeconstantly and not only when a tachycardia condition prevails. Usually,the term overdrive rate is used to denote a rate which is above areference rate, namely, in known cardioverters, above a tachycardiarate. For the pacemaker claimed herein, the reference rate for theclaimed permanent overdrive stimulation is not a pathological rate but anatural intrinsic rate, which for example, corresponds to a healthyatrial heart rate or which is derived from a per se known manner fromparameters or measurement values characterizing the physiological demandof a patient. The terms overstimulation rate or overdrive stimulationrate in the sense used herein are not to be confused with the knownoverdrive stimulation rate which is used for tachycardia treatment.Overdrive stimulation rate in the conventional sense means a rate whichis greater than a tachycardia rate while what is meant here is a ratewhich is only slightly greater than a healthy, physiologically adequaterate and at any event much less than a tachycardia rate.

The last-mentioned case concerns a rate-adaptive pacemaker in which astimulation rate for the cardiac pacemaker is so set that thestimulation rate depends on the physiological demand of a patient, thusfor example rises with increasing effort, as is also the case with ahealthy heart.

In accordance with the concept of permanent overstimulation in analternative embodiment the control unit of the implantable cardiacpacemaker is adapted to predetermine a fixed stimulation rate of between70 and 80 stimulation pulses or beat per minute, in particular a rate ofabout 80 per minute. Such a stimulation rate is between about ten andtwenty beats per minute above a natural heart rate of a patient in therest condition.

In a preferred configuration the control unit of the implantable cardiacpacemaker is adapted to automatically form the overstimulation rate(overdrive stimulation rate) for permanent overstimulation. For thatpurpose, it is particularly provided that the control unit is adapted topredetermine the overstimulation rate in direct or indirect dependenceon transconductions of atrial stimuli by way of an AV node of a heartfrom the atrium to the ventricle of the heart, in such a way that thenumber of transconductions or the number of transconducted stimuliwithin a predetermined time, or in relation to a predetermined number ofventricular events, does not exceed a predetermined degree. Theventricular events can be either stimulated or natural events. Thecontrol unit is thus so designed that, by evaluation of the events orother measurement values, in a retrospective period of time, itautomatically ascertains a patient-individual, optimum overstimulationbase rate. The term overstimulation base rate means herein theoverstimulation rate to be provided in the rest condition of thepatient. That can be increased in the context of rate adaptation in thecase of increased physical effort on the part of the patient.

The choice of the retrospective period of time, in accordance with theabove-mentioned predetermined time or the predetermined number ofventricular events, can preferably be variably predetermined.

In order to achieve adaptation of the overstimulation rate, which is asfast as possible, it is preferably provided that the overstimulationrate is already to be increased when only a single transconductionphenomenon or a single transconducted stimulus is detected by thecontrol unit. In that case, the control unit is so designed that itautomatically increases the overstimulation rate as soon astransconduction or a transconducted stimulus is detected.

Preferably, the control unit increases the overstimulation rate only fora given period of time and thereafter it decreases it again. Thatensures that an excessively high overstimulation rate is not permanentlyset. On the other hand, the number of possible transconductions iseffectively reduced. In an alternative preferred embodiment, the controlunit is adapted to increase the stimulation rate as soon as between tenand twenty percent of transconductions or between ten and twenty percentof transconducted stimuli in relation to a total number of ventricularevents are detected in the previous period of time. In other words, thecontrol unit sets an increased stimulation rate as soon as the totalnumber of detected ventricular events in a retrospective period of timeinvolves between 10% and 20% of such events which are based ontransconductions.

In connection too with the last-mentioned, alternatively preferredembodiment, it is preferably provided that an increase in theoverstimulation rate is reversed again after a predetermined time,possibly in steps.

In a further alternative embodiment, the control unit is adapted to setthe overstimulation rate in dependence on the number of episodes ofventricular tachycardia within a predetermined period of time or inrelation to a predetermined number of ventricular events. An advantageof this configuration over alternative configurations is in particularthat tachycardia episodes can be easily detected.

Preferably, the control unit in accordance with the last-mentionedconfiguration is adapted to increase the variable stimulation rate ifthe number of episodes of ventricular tachycardia exceeds apredetermined limit value. In a particularly preferred configuration,that predetermined limit value is 5 percent of episode-ventriculartachycardia in relation to the total number of ventricular events in aretrospective observation period of time.

It is preferably also provided in this last-mentioned configuration,that the overstimulation rate is reduced again after an increase. As inall the above-mentioned cases, the gradual reduction in the increasedoverstimulation rate can take place after a predetermined time in onestep or in a plurality of steps after a respective predetermined time.In that case, the time factors can be specified in minutes or hours orhowever, can also be in the form of a predetermined number of cardiaccycles.

In a particularly preferred variant, the control unit is connected to asensor with which a measurement value dependent on the physical activityof a patient is to be ascertained. In that case, the control unit isadapted, in dependence on the measurement value, to ascertain astimulation rate adapted to the physiological demand and to set asuitable overstimulation rate which is above same.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail by means of anembodiment with reference to the drawings in which:

FIG. 1 shows a perspective view of a dual-chamber cardiac pacemaker withconnected and implanted atrial and ventricular electrodes, and

FIG. 2 shows a roughly schematic block diagram of a cardiac pacemakeraccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 diagrammatically shows a human heart 10, into the ventricle ofwhich is inserted a ventricular electrode line 12 and into the atrium ofwhich is inserted an atrial electrode line 14. The ventricular electrodeline 12 has a ventricular tip electrode 16 and a ventricular ringelectrode 18. The atrial electrode line 14 has an atrial tip electrode20 and an atrial ring electrode 22. The electrode lines 12 and 14 areconnected to a dual-chamber cardiac pacemaker 24.

FIG. 2 shows the components of the cardiac pacemaker 24. These are abipolar electrode line connection 26 for a ventricular electrode lineand a unipolar or bipolar electrode line connection 28 for an atrialelectrode line. The illustrated simple embodiment of the cardiacpacemaker 24 further has a ventricular detection unit VS and aventricular stimulation unit VP which are connected by way of theelectrode line connection 26 with a ventricular electrode line to theelectrode line 12 in FIG. 1. The ventricular detection unit VS servesfor recording electrical signals of the heart, for example anintracardiac electrocardiogram. The ventricular stimulation unit VSincludes a stimulation pulse generator with associated energy storagedevices, charge pump etc as is known from the state of the art, and issuitable for generating and, if required, delivering ventricularstimulation pulses. For triggering ventricular stimulation pulses, theventricular stimulation unit VS is connected to a control unit 30. Thecontrol unit 30 is also connected to the ventricular detection unit VSand to an atrial detection unit AS. The atrial detection unit AS is tobe connected by way of the electrode line connection 28 to an atrialelectrode line like the electrode line 14.

For communication with external devices, the control unit 30 is providedwith a telemetry unit Rx/Tx for the wireless communication, for example,of intracardially recorded electrocardiograms from the cardiac pacemaker24 to an external device or for the transmission of programming commandsfrom an external device to the cardiac pacemaker 24.

The drawing does not show an atrial stimulation unit which ispotentially to be provided, and which enables the cardiac pacemaker 24also to stimulate the atrium of a heart. Such a dual-chamber cardiacpacemaker in the narrower sense can be operated, for example, in the perse known DDD mode. Means (not further shown) for detecting intracardiacimpedance can be used in such a cardiac pacemaker on the one hand fordetecting stimulation success (capture detection) immediately after thedelivery of a stimulation pulse so that, in the case of stimulationsuccess possibly failing to occur, a back-up pulse can be triggeredimmediately. In the present case, the per se known means forascertaining the physiological demand of a patient in the case ofphysical effort will not be further discussed in greater detail. Suchmeans can be, for example, a blood oxygen sensor or again, an impedancesensor which derives a control signal for the heart rate from theintegral of an intracardiac impedance pattern so that the heart rate, inthe sense of a closed regulating circuit (closed-loop stimulation), canbe adapted to the physiological demand.

What is of interest in the present case is, in particular, a VVI mode inwhich the cardiac pacemaker 24 can be operated. The stimulation rate,which is to be used as the basis for that mode of operation, can becontrolled in the sense of a rate-adaptive cardiac pacemaker independence on the physiological demand or it can be a fixedlypredetermined base rate. A rate-adaptive cardiac pacemaker 24 however,is preferred. That operation of determining physiological demand, whichis required for that purpose, can be effected, for example, asindicated, by means of a blood oxygen sensor or by evaluation of anintracardiac impedance pattern recorded by means of suitable sensors. Inthat case, the sensors can be the electrodes 16 through 22, includingthe housing of the cardiac pacemaker 24. Evaluation of the intracardiacimpedance pattern is effected in the control unit 30.

In addition, depending on the respective patient, it is possible for ahealthy intrinsic heart rate in the atrium of the heart 10 to bedetected by means of the atrial detection unit AS.

For the VVI mode of operation of the cardiac pacemaker 24, which is ofinterest here, firstly a base rate is ascertained on the basis, forexample, of an atrial heart rate detected with the atrial detection unitAS, or by calculation based on the physiological demand of the patient.The control unit 30 is then so designed as to form from the variablebase rate, a variable stimulation rate as an overstimulation rate, whichis for example between 10 and 20 beats per minute higher than the baserate adapted to the physiological demand. The control unit 30 ascertainsthe precise value of the overstimulation rate on the basis of detectedstimulus transconductions from the atrium to the ventricle. Suchstimulus transconductions can be ascertained for example by evaluationof the signals recorded in the atrium by means of the atrial detectionunit AS, in comparison with corresponding signals detected in theventricle by means of the ventricular detection unit VS. Theoverstimulation rate to be ascertained by the control unit 30 isincreased if, in a predetermined retrospective period of time (forexample 20 cardiac cycles), more than 10% or 20% stimulustransconductions from the atrium to the ventricle are detected. If,conversely, no stimulus transconductions are detected in thatretrospective period of time, the overstimulation rate is graduallyreduced. Reducing the overstimulation rate increases the probability oftransconduction of atrial stimuli to the ventricle. As soon as apredetermined number of transconductions is detected in thepredetermined retrospective period of time, the overstimulation rate isincreased again. In that way, transconductions of stimuli from theatrium to the ventricle are naturally substantially prevented and thusalso, as far as possible, the occurrence of ventricular tachycardias.

1. An implantable cardiac stimulator, in particular a cardiac pacemakeror cardioverter/defibrillator (ICD), comprising a ventricular detectionunit (VS) which is to be connected to an intracardiac electrode and isadapted to record and detect ventricular events, and a ventricularstimulation unit (VP) which is to be connected to a ventricularelectrode and is adapted to produce ventricular stimulation pulses fordelivery to the ventricle of a heart, and a control unit which isconnected to the ventricular detection unit (VS) and to the ventricularstimulation unit (VP) and is adapted to actuate the ventricularstimulation unit (VP) in a VVI mode in ventricle-inhibited fashion insuch a way that a ventricular stimulation pulse is triggered at a momentin time predetermined by a stimulation rate if it is not inhibited bydetection of a natural ventricular contraction by means of theventricular detection unit (VS) within a predetermined time window,characterized in that the control unit is adapted to predetermine astimulation rate which is higher than an in particular intrinsic rateappropriate to the physiological demand.
 2. A cardiac stimulator as setforth in claim 1 characterized in that the control unit is adapted topredetermine a fixed stimulation rate of between 70 and 90 per minute,preferably about 80 per minute.
 3. A cardiac stimulator as set forth inclaim 1 characterized in that the control unit is adapted topredetermine a variable stimulation rate in dependence on indirectly ordirectly detected transconductions of atrial stimuli by way of an AVnode of a heart from the atrium to the ventricle of the heart, in such away that the number of transconductions or the number of transconductedstimuli within a predetermined time or in relation to a predeterminednumber of ventricular events does not exceed a predetermined degree. 4.A cardiac stimulator as set forth in claim 3 characterized in that thecontrol unit is adapted to increase the variable stimulation rate if thenumber of transconductions or the number of transconducted stimuliexceeds the predetermined degree.
 5. A cardiac stimulator as set forthin claim 4, wherein the predetermined degree is a single transconductionor a single transconducted stimulus.
 6. A cardiac stimulator as setforth in claim 4, wherein the predetermined degree is between 10 and 20%of transconductions or between 10 and 20% of transconducted stimuli inrelation to a total number of ventricular events.
 7. A cardiacstimulator as set forth in claim 6, wherein the control unit is adaptedto set a ventricular stimulation rate in dependence on the number ofepisodes of ventricular tachycardia within a predetermined period oftime or in relation to a predetermined number of ventricular events. 8.A cardiac stimulator as set forth in claim 7, wherein the control unitis adapted to increase the variable stimulation rate when the number ofepisodes of ventricular tachycardia exceeds a predetermined limit value.9. A cardiac stimulator as set forth in claim 8, wherein thepredetermined limit value is 5% of episodes of ventricular tachycardiain relation to the total number of ventricular events.
 10. A cardiacstimulator as set forth in claim 1, wherein the control unit is adaptedto form an overstimulation rate as a variable stimulation rate from anatrially detected intrinsic base rate or a physiologically adequate baserate ascertained by detection and evaluation of a physiological demandof a patient, in such a way that the overstimulation rate is higher thanthe base rate by a difference rate.
 11. A cardiac stimulator as setforth in claim 10, wherein the control unit is adapted to set thedifference rate in dependence on the number of detected stimulustransconductions from the atrium to the ventricle within a predeterminedretrospective period of time, in such a way that the control unitincreases the difference rate if the number of detected stimulustransconductions exceeds a predetermined degree.
 12. A cardiacstimulator as set forth in claim 4, wherein the control unit is adaptedto gradually reduce the variable stimulation rate until an increase inthe variable stimulation rate occurs again.
 13. A cardiac stimulator asset forth in claim 8, wherein the control unit is adapted to graduallyreduce the variable stimulation rate until an increase in the variablestimulation rate occurs again.
 14. A cardiac stimulator as set forth inclaim 10, wherein the control unit is adapted to gradually reduce thevariable stimulation rate until an increase in the variable stimulationrate occurs again.
 15. A cardiac stimulator as set forth in claim 3wherein the predetermined degree is a single transconduction or a singletransconducted stimulus.
 16. A cardiac stimulator as set forth in claim3 wherein the predetermined degree is between 10 and 20% oftransconductions or between 10 and 20% of transconducted stimuli inrelation to a total number of ventricular events.
 17. A cardiacstimulator as set forth in claim 1, wherein the control unit is adaptedto set a ventricular stimulation rate in dependence on the number ofepisodes of ventricular tachycardia within a predetermined period oftime or in relation to a predetermined number of ventricular events. 18.A cardiac stimulator as set forth in claim 2, wherein the control unitis adapted to set a ventricular stimulation rate in dependence on thenumber of episodes of ventricular tachycardia within a predeterminedperiod of time or in relation to a predetermined number of ventricularevents.
 19. A cardiac stimulator as set forth in claim 3, wherein thecontrol unit is adapted to set a ventricular stimulation rate independence on the number of episodes of ventricular tachycardia within apredetermined period of time or in relation to a predetermined number ofventricular events.
 20. A cardiac stimulator as set forth in claim 4,wherein the control unit is adapted to set a ventricular stimulationrate in dependence on the number of episodes of ventricular tachycardiawithin a predetermined period of time or in relation to a predeterminednumber of ventricular events.